fmap is a shortcut to refer to the image loaded in the memory of the OBI Presentation Server.
And then not of the application server.
.
This images are loaded during the initialization of the server and are located in the style path:
The paths are dependent of your configuration.
It's used to prevent a cross-site scripting (XSS).
If you have a image located here (and also exposed in the application or web server)
You must refer to it in OBI Presentation service as :
fmap:mydirectory\myimage.jpg
You must restart the BI Presentation Service in order to make your skin files available to fmap.